Occurs immediately after the construction ofstructure.Also known as ‘Distortion Settlement’, it occursdue to distortion in foundation soil.Although its not truly elastic, it is computedusing elasticity theory. It is denoted by Si.
Occurs due to gradual dissipation of porepressure induced by external loading andconsequently expulsion of water from the soilmass, hence volume change.Important for Inorganic clays. This component isdetermined using Terzaghi’s theory ofconsolidation.It is denoted as Sc.
This settlement occurs after completion of theprimary consolidationOccurs at constant settlement effective stresswith volume change due to rearrangement ofparticles.This component of the settlement is due tosecondary consolidation. It is significant fororganic soils.Determined from the coefficient of secondaryconsolidation. Denoted as Ss
For any of the mentioned settlementcalculations, we first need vertical stress increasein soil mass due to net load applied on thefoundation.The total settlement (S) is given as S = S s + Sc + Si
Underground erosion may cause formation ofcavities in the subsoil which when collapse causesettlement.Structural collapse of soil may cause todissolution of material responsible for the inter-granular bond of grains.Temperature change causes shrinkage inexpansive soils due to which settlement occurs.
Frost heave occurs if the structure is notfounded below the depth of frost penetration.When thaw occurs, settlement occurs.Vibrations and Shocks cause large settlement,especially in loose and cohesionless soils.Mining subsidence of ground may occur due toremoval of minerals and other materials frommines below.
Land slides occur on unstable slopes, there maybe serious settlement problem.If there are changes due to construction of a newbuilding near the existing foundation, thesettlement may occur due to increase in thestress.
Dead loadLive loadWind loadSnow loadEarth pressureWater pressureEarthquake load
Dead load : The dead load include the weight of materials permanently fixed to the structure, such as beam, column, floors,walls and fixed service equipment. Can be calculated if sizes and types of structural materials are known. There is a problem in estimating the self weight of the structure because self weight is initially assumed and the structure is designed. Then weight is calculated from the designed dimension and compared with the assumed weight.
Live load : The live loads are the movable loads that are not permanently attached to the structure. These loads are applied during a part of its useful life. Loads due to people, goods, furniture, equipment etc. are considered in live loads. It is difficult to estimate the live load accurately. These are specified by local building codes.
Wind load : Wind loads acts on all exposed surfaces of the structure. This loads depends on the velocity of wind and type of the structure. Wind loads are specified by building codes.Snow loads : Snow loads are occur due to accumulation of snow on roofs and exterior flat surfaces in cold climates. The unit weight of snow load is usually taken as 1kN/sq m
Earth pressure : Earth pressure produce lateral force against the structure below the ground surface or fill surface. The earth pressure is normally treated as dead load.Water pressure : Like earth pressure water, water pressure also produces a lateral force against the structure below the water level.Earthquake pressure : The force due to earthquake may be vertical, lateral or torsional on a structure in any direction.
Dead load and live load are computed bytributary area method.In tributary area method, it is assumed that acolumn carries all the load in the floor areaenclosed by lines equidistant from its adjacentcolumns.Live loads are temporary and transient. A part ofit may act for a duration that may inducedsettlement, especially for cohesive soil.Live loads are referred from IS-875.
It is assumed that wind load and earthquake loaddo not act simultaneously.According to IS 1904-1978 foundation should beproportioned for the combinationdead load + live load anddead load + live load + wind load or seismicload
If the wind load is less than 25% of the total Deadload + Live Load, then it may be neglected and itis designed for the combination of Dead load andlive load only.The Foundation pressure should not the exceedthe safe bearing capacity by more than 25% forthe combination of dead load, live load and windload.
Settlement under loadsSettlement due to other causes